Rules integration: I cannot set it to work with Membership Fields

Hi there,

I have some OG groups in my site. Each group is an online classroom.

The membership has a few fields (I use the default membership), and one of those fields is a select field with "approved" or "not approved" information about each student's final exam.

I would like to use rules to send an email to each student that change that OG membership field status to "approved". I want this workflow only for users of "exchange-student" role.

But I cannot do that because it seems that I didn't fetched the OG membership. I don't know what I'm doing wrong here.

This is my Rule:

Event:

After updating an existing og membership
After updating an existing user account

Conditions:

Entity has field: (Parameter: Entity: [og-membership], Campo: field_examen)
Data comparision: Parameter: Data to compare: [og-membership:field-examen, Data value: Aprobado

Action:

Fetch entity by id: Parameter: Entity type: Usuario, Identifier: [og-membership:id]
Provides variables: Membresía Alumno (membresia_alumno)
Send mail: Parameter: Para: [membresia-alumno:mail], Asunto: Aprobaste!, Mensaje: Hey, aprobaste!, Idioma: [membresia-alumno:language]

My problem is this rule won't send anything (it will send the email if I replace the parameter [membresia-alumno:mail] with a hardcoded email address).

What am I doing wrong? What am I missing?